Bias trouble.(very popular topic subject)

[BobW]

Gary, PM or call me (ampman).

[BobW]

Also, the 1 ohm resistors from the cathodes must join pins 1 & 8 with the resistor going to ground.

Pin 8 is the cathode for 6l6GC, pin 1 is not connected to anything in the tube. Pin 1 is used only for EL34 .

I certainly agree.
Geary and I have emailed back and forth a few times, and he wants the option to use EL34's, hence the 1,8 pins tied to ground via the 1 Ohm. He has managed to gut and rebuild a Carvin on his own, only asking for help when needed. He's a welcome member to this forum.

[Structo]

Also, the 1 ohm resistors from the cathodes must join pins 1 & 8 with the resistor going to ground.

Pin 8 is the cathode for 6l6GC, pin 1 is not connected to anything in the tube. Pin 1 is used only for EL34 .

Yeah I know I just get in the habit of wiring it that way in case I do want to run EL34's.

[garysanders]

I am getting over to the bench.Be back soon with results.Thanks again for the tips!Right back.

[garysanders]

OK the pin 5 V is -42.I changed the diode and resistor,which were in the right direction but one of them could have been DOA because I am getting a good -V.The current through the 1R to pin 8 is overkill,over 150 mA.I didnt see how high it got,killed it when it got that high.So Im getting little or no resistance on the bias?I didnt let it go near that long with the tubes in it.That would have set the smoke alarms off.

I'm stumped.Unless I am looking at the pot and have it backwards but that shouldnt matter,right?I put plastic washers on both sides of the pot to make sure nothing was shorting out.What direction could I go with the resistor.(27K for now).I measured my "10K" pot and found it to be 7K5....

[Structo]

How do you have the 27K resistor grounded?

[butwhatif]

OK the pin 5 V is -42.I changed the diode and resistor,which were in the right direction but one of them could have been DOA because I am getting a good -V.The current through the 1R to pin 8 is overkill,over 150 mA.I didnt see how high it got,killed it when it got that high.So Im getting little or no resistance on the bias?I didnt let it go near that long with the tubes in it.That would have set the smoke alarms off.

I'm stumped.Unless I am looking at the pot and have it backwards but that shouldnt matter,right?I put plastic washers on both sides of the pot to make sure nothing was shorting out.What direction could I go with the resistor.(27K for now).I measured my "10K" pot and found it to be 7K5....

My guess is the tubes are cooked. Got a set of known good ones u don't mind putting at risk? There is a 1R resis on each cathode right? -that's the reading for all, or just one? and You need to get that -42 closer to -52 v.

[garysanders]

How do you have the 27K resistor grounded?

Yes,just like the "Best 124 Layout" layout

My guess is the tubes are cooked. Got a set of known good ones u don't mind putting at risk? There is a 1R resis on each cathode right? -that's the reading for all, or just one? and You need to get that -42 closer to -52 v.

That is the reading for ONE


As for the -42 to -52,I figured I would have to tweak some things like this because I used the Carvin PT.

[Structo]

So you said that you have the bias pot isolated from the chassis with washers.

If the resistor is grounded to the pot how is it reaching chassis ground?

Know what I'm getting at? That 27K resistor must see chassis ground.

[garysanders]

It has a short ground wire going to a terminal bolt to ground.Just like the Best 124 Layout image.

[butwhatif]

It has a short ground wire going to a terminal bolt to ground.Just like the Best 124 Layout image.

if u have -42 on the bias pot output, what is the input bias voltage to the pot?

[heisthl]

I did a Carvin conversion a while back and I think this is what ended up being the final bias config:

[garysanders]

I did a Carvin conversion a while back and I think this is what ended up being the final bias config:

Oh,theres nothing Carvin in there any more.I clipped all the wires and chunked the PCB in the parts box.The only thing Carvin left is the logo printed on the back of the chassis.I used the transformers and the rest is wannabee dumblebee.Well,when it gets to working it is.

I have the X100B head and have it biased similar as in your post.Its a great amp but I needed something with NO op amps,so I heard BobW's Dumbo and went with that.

AND I have made a little improvement on my trouble but I'll get back to posting later.Im making blueberry cheescake muffins at the moment and have a lesson to give shortly.Be back after all that.

[Bob-I]

This seems exceptionally strange. Are you measuring the same ground point for the 1 ohm resistors and the bias voltage? For the tubes to run away like this, the only explaination is there's no -bias voltage. If the bias is not grounded you may see -42 from the pot to pin 5, but not ground to pin 5.

How about posting some pics?

[talbany]

Don't know if this helps but as it relates to the subject.. Marshall style bias supply...

The following graph shows the grid bias voltage versus the ratio RS / RL for the simplified grid bias supply schematic. The curves are for five different transformers: 300-0-300 (upper curve), 325-0-325, 350-0-350, 375-0-375, and 400-0-400 (lower curve).


The ratio of source to load resistance varies from 5 (variable resistor set to minimum) to 3.3 (resistor set to maximum). The middle curve, corresponding to 350-0-350 power transformer, shows that the grid bias voltage can be adjusted between -28 and -42 volts. The ultimate adjustment range will vary depending on the error tolerance of the components and variations in AC line voltage.

Tony VVT